1. Field of the Invention
The invention relates to internal combustion engine controllers and in particular to an engine speed controller employing an electro-mechanical actuator.
2. Background of the Art
The precise speed control of internal combustion engines is desired for many applications but is particularly important when such engines are used to drive AC generators. The speed of the engine determines the frequency of the generated power and many AC powered electrical devices require accurately regulated frequency. In addition, this accurate speed control must be maintained under rapid load variations which may result from nearly instantaneous changes in the consumption of electrical power from the generator. Variation in engine speed with change in engine load is termed "droop".
Engine speed control may be performed by a number of methods. A mechanical governor may sense the rotational speed of the engine and open or close the throttle to regulate the engine speed in response to imputed load changes. Such mechanical control has the advantage of being relatively inexpensive, but may allow substantial droop during normal load variations.
More sophisticated engine speed control may be realized by sensing engine speed electrically and using an an electromechanical actuator connected to the throttle to change the throttle position. Typically, the electro-mechanical actuator is a linear or rotary actuator. As the names imply, a linear actuator has a control shaft which extends from the body of the actuator and moves linearly by a distance proportional to the magnitude of a current or voltage applied to the actuator. A rotary actuator has a shaft which rotates by an angle proportional to the magnitude of the applied current or voltage. In both actuators, a spring returns the shaft to a zero or "home" position when no voltage or current is applied to the actuator. The power consumed by these actuators is increased by this return spring whose force must be constantly overcome.
The power required by the use of a return spring increases the cost and weight of a throttle control using a linear or rotary actuator. For this reason, it is known to use a bidirectional stepper motor in place of a linear or rotary actuator for the purpose of electronic engine control.
A bidirectional stepper motor is an electro-mechanical device that moves a predetermined angular amount and direction in response to the sequential energizing of its windings. With such a bidirectional stepper motor, the return spring may be omitted or made weaker allowing the use of a smaller motor with equivalent or better dynamic properties than the linear or rotary actuators.
The use of a lower powered bidirectional stepper motor typically requires that a position sensing device be attached directly to the throttle. The reason for this is that the stepper motor may have a arbitrary orientation when its power is first applied and hence the position sensing device is necessary to provide an absolute indication of the throttle position. Such position sensing devices add complexity to the throttle and increase its cost.